The identification and characterization of MHC linked class Ib loci has proven to be a exciting area of investigation. Studies from our laboratory and others have shown that class Ib molecules can function to present peptide and non-peptide epitopes to T cells. Several class Ib molecules have been implicated in the immune response to intracellular bacterial pathogens and mouse/human counterparts have been identified. Furthermore, class Ib molecules have also been found to serve as ligands for NK cells. This information indicates that this subset of MHC class I molecules have evolved to play key roles in the immune process. We propose to design a new murine model for the analysis of mouse and human class Ib function. We will engineer, characterize and express a single chain Qa-1b class Ib molecule as a transgene in class I deficient mice. This mouse model will allow the definition of the role for Qa-1 in the selection of the T cell repertoire. Also we will be able to examine the range of pathogens for which Qa-1 restricted CD8+ effector cells are evoked. We will investigate the role of Qa-1 as a ligand for inhibitory receptors expressed on NK cells as well as CD8+ T cells. We hypothesize that Qa-1 has unique structural features that allow it to both serve as a ligand for both NK receptors and antigen-specific T cell receptors. Lastly, we propose to study the structure of endogenous self-peptides bound to Qa-1 and the physical/chemical definition of ligand binding to Qa- 1. Such studies will allow us to fully understand the peptide features requisite for interaction with the Qa-1 binding site. These studies will not only lend insight into the role of Qa- 1 in the presentation of self peptides to alloreactive T cells, but also facilitate the identification of bacterial and/or TCR/Ig derived peptides presented by Qa-1 to regulatory CD8+ T cells. Also, novel antigen presentation pathways may be revealed. Collectively such studies will provide the basic principles needed for our long term objective, to utilize this novel family of non-polymorphic class I molecules as targets for vaccine strategies and/or immune modulation.